1. Field Of The Invention
The present invention relates generally to downhole pumps for testing of wells, and more particularly, to a downhole progressive cavity type pump.
2. Description Of The Prior Art
After a well has been drilled it is common practice to test the production capabilities of the subsurface producing formations so that the well completion may be properly designed and constructed.
A problem is often encountered in that some subsurface formations do not have enough reservoir pressure to produce well fluids to the surface against a full column of fluid in the well. These wells pose a problem in their analysis during testing because the time interval over which the well will naturally flow is not long enough to determine the full reservoir parameters.
One common method for testing these wells is to utilize a downhole pump of the jet pump type. The jet pump uses a separate fluid pumped down the well annulus and ported into the testing string to raise the formation fluids by a jetting action. An example of such a jet pump is the SSJ.TM. jet pump available from Trico Industries, Inc., of Huntington Park, Calif. Such a jet pump system is not ideal because of the power requirements and because of problems created by the mixing of the power fluid and the produced formation fluids.
Another pump which has been utilized as a downhole formation pump in a test string is a progressive cavity type pump. SPE Paper No. 9607, "Use of a Down-hole Mud Motor as a Pump for Drill-Stem Testing", Cobbett, 1981, discloses the use of a progressive cavity pump in a drill stem test string. This progressive cavity pump has its rotor permanently in place within the stator, and the test string does not include a tester valve since well fluid cannot freely flow through the test string. The only manner in which well fluid can flow through the test string is by operation of the progressive cavity pump which blocks the flow passage through the test string.